The present invention relates to an ink jet recording apparatus which uses an ink jet head having a number of ink nozzles arranged and drives the ink jet head via an image signal to cause selectively the ink nozzles to eject droplets of ink onto paper for recording.
One ink jet recording apparatus is an on-demand ink jet printer. With such a type of ink jet printer, it is important to increase the speed of operation. The more ink nozzles in the ink jet head, the faster we can make printing. A technique to increase the number of ink nozzles is forming a long ink jet head by combining integrally a plurality of head units each having a large number of ink nozzles arranged in a line with the arrangement of the ink nozzles in each head unit substantially parallel with that in every other head unit. A line head is formed by making the head length equal to the width of recording paper, permitting fast printing.
The head unit is composed, as shown in FIG. 29, of a nozzle section 2 provided with a large number of ink chambers 1, a body section 4 provided with a common ink chamber 3 for supplying ink to each ink chamber 1, and an ink supply pipe 5 for supplying ink to the common ink chamber 3. Causing a volume change to occur in each ink chamber 1 allows a droplet of in ejected from a corresponding one of ink nozzles 6, thereby making dot printing. The vacant ink chamber 1 is replenished with ink by the common ink chamber 3.
Systems for causing a volume change to occur in the ink chambers 1 include piezoelectric control systems that make use of deformation of piezoelectric members and-heat control systems that make use of heat generated by heating elements. The use of such control systems to cause a volume change to occur in each ink chamber in an arbitrary fashion allows printing on demand.
At the time of ink ejection, the volume change in an ink chamber 1 is also transferred to the common ink chamber 3 in the form of a change in pressure. As a result, ink chambers 1 situated at the head ends will be greatly affected by the common ink chamber 3 when a number of ink chambers eject ink, so that the volume of ink ejected from a certain number of ink chambers situated near and at the head ends increases.
In the case of an ink jet head consisting of a single head unit, even if the volume of ink ejected from a certain number of ink chambers at the end portions of the head changes -and consequently there is some-change in printing density, the resulting irregularities in density is difficult to notice because the change in printing density occurs at the ends of a line of printing. In contrast, in the case of an ink jet head in which a plurality of head units are arranged in the direction of their length, the ends of head units adjacent to each other are located midway on a line of printing. The irregularities in the density in those portions will appear noticeably in the form of lines.
For example, as shown in FIG. 30, when an ink jet head in which head units 7 and 8 are arranged in the direction of their length in non-overlapping fashion is driven by an image signal at a fixed level, the volume of ink projected from ink nozzles 7a and 8a in the boundary portion between the head units increases, resulting in a sharp increase in printing density from d0 to d1. This produces noticeable irregularities in printing density in the form of a line.
It is an object of the present invention to provide an ink jet recording apparatus using an ink jet head assembly constructed from a plurality of head units arranged such that every two adjacent head units overlap with each other, which allows irregularities in printing density resulting from variations in the volume of ejected ink from ending ink nozzles of each head unit to be reduced and irregularities in printing density in line form extending in the direction in which a recording medium moves to be reduced.
According to the present invention, there is provided an ink jet recording apparatus comprising: an ink jet head formed from a plurality of head units each having a large number of ink nozzles arranged in a line, the head units being arranged such that they are substantially parallel to one another in the direction of arrangement of their ink nozzles and, in the direction in which a recording medium moves, every two adjacent head units overlap with each other by a number Q of ink nozzles including a number P (Q greater than Pxe2x89xa71) of ending ink nozzles that may cause a change in printing density; and drive means responsive to an input image signal for driving the ink jet head to cause selectively the ink nozzles to eject ink onto the recording medium for printing while moving the recording medium in a direction perpendicular to the line direction in which the ink nozzles of the respective head units are arranged, the driving means controlling the ink jet head so that one dot is formed by ink ejection from each ink nozzle of one head unit in the overlapping zone of the adjacent head units and the corresponding one of the other head unit and the ratio in the amount of ejected ink of one to the other of overlapping ink nozzles is made variable so that the average printing density in the overlapping zone of the head units varies at a fixed rate.
Thus, the present invention allows an ink jet head assembly constructed from a plurality of head units arranged in the direction of their length to reduce irregularities in printing density resulting from variations in the volume of ejected ink from ending ink nozzles of each head unit. In addition, irregularities in printing density in line form extending in the direction in which a recording medium moves can be reduced, thereby further improving the recording quality.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.